Add documentation for --fernignore flag in fern generate command

[devin-ai-integration]

Summary

Documents the new --fernignore flag for the fern generate command, which allows users to specify a custom .fernignore file during remote SDK generation. This flag was added in fern-api/fiddle#537.

The documentation includes:

• Description of the flag's purpose, including that Fern commits the new .fernignore to the repository as part of generation
• Usage example
• Note clarifying it only works with remote generation (not --local or --runner)
• Flag added to the top-level command syntax example

Updates since last revision

• Clarified that Fern will commit the new .fernignore to the repository as part of the generation process (per user feedback)
• Resolved merge conflict and restored the Note about remote-only generation

Review & Testing Checklist for Human

• Verify the description accurately reflects the behavior - specifically that Fern commits the new .fernignore during generation (derived from reading fiddle PR code, not tested)
• Check that the documentation style matches other flags in the same section
• Confirm the Note about remote-only generation is still accurate

Notes

• Could not run Vale locally as it wasn't installed in the environment
• Documentation based on code review of the CLI implementation in fern-api/fern and the fiddle PR

Link to Devin run: https://app.devin.ai/sessions/92dde660f075401f91be2ed267c45f55
Requested by: Fern Support (info@buildwithfern.com)

[devin-ai-integration]

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